This invention relates to speech signal communication systems and more particularly to digital speech signal processing adapted to reduce transmission rates.
The processing of signals for transmission over digital channels in telephone and other types of communication systems generally includes the sampling of the input signals, quantization of the samples, and generation of a set of codes representative of the quantized signal samples. As is well known, speech signal samples are highly correlated so that each signal sample has a component that is predictable from its past values. The predictable and unpredictable components of the speech signal can be separated and encoded at approximate rates to provide efficient utilization of a digital channel without affecting the subjective quality of the speech signal.
Predictive signal arrangements as disclosed in U.S. Pat. Nos. 3,502,986 and 3,631,520 involve generation of predictive parameter signals from the succession of speech samples and the formation of a predicted value for each speech signal sample from the generated parameters and the preceding speech signal samples. The difference between each sample and its predicted value is quantized, digitally encoded, and sent to a receiver along with the predictive parameters. The difference signal is encoded at the receiver and combined with the predictive parameters and other signal information already stored in the receiver. In this manner, only the unpredicted signal component need be quantized and transmitted at a high bit rate and a saving in channel capacity is achieved. The saving is generally reflected in the reduced bit rate needed to transmit a signal of predetermind quality.
U.S. Pat. No. 4,133,976 issued Apr. 7, 1978 to B. S. Atal and M. R. Schroeder and assigned to the same assignee discloses a predictive speech signal coding arrangement in which a set of formant-related predictive parameters corresponding to the short-term redundant structure of the speech signal and a set of pitch-related prediction parameters corresponding to the long-term redundant structure of the speech signal are generated. Since the speech signal is quasi-stationary, the prediction parameters need only be generated once every 10 milliseconds. The remaining portion of the speech signal corresponds to the unpredicted component generally termed the prediction residual.
While the prediction parameter signals representative of the predictive speech signal component can be transmitted at a relatively low bit rate without adverse effects, the transmission rate of the prediction residual is critical to the quality of the reconstructed speech signal. Typically, the predicted signal component parameters require a transmission rate of 3 to 4 kilobits per second. At total bit rates lower than 10 kilobits per second, it is often necessary to quantize the prediction residual with only one bit per sample. This two level quantization results in both peak-clipping of the prediction residual and granular distortion. It is an object of the invention to provide improved digital speech communication at low bit rates.